For the demands of lightweight metal structural parts in automobile and spaceflight system, the joining of aluminum to steel and titanium gains its prospects. Because of the enormous differences in the physical-chemical properties between them, the main problem is the formation of brittle IMCs. Researchers have illustrated that fusion welding-brazing methods, which are suitable for various welding joint types, easy to be operated automatically and can control the growth of brittle compounds, are the most adaptive ways to join dissimilar metals. However, the fusion welding-brazing seams produced by arc or laser heat source are prone to oxidation because of the poor protection of the gas shielded atmosphere. It is necessary to add fluxes to remove the oxide films on base metal surfaces. Due to vacuum protection atmosphere, as well as low heat-input and rapid cooling rate, electron beam welding was considered as the most suitable method for the welding of dissimilar metal joints . In this study, electron beam welding-brazing of aluminum to stainless steel and titanium with no flux using an Al-based filler wire was conducted. The interface structures of these two joints were discussed. The regulating mechanism of the interfacial morphology was deduced.
304 stainless steel (0Cr18Ni9) and 1050 commercial pure aluminum (CP-Al) plates and an AlSi5 wire with a diameter of 0.8 mm were selected as the base metals and filler metal for the joining of aluminum/steel. TA2 pure titanium and 2024Al alloy (Al4.5Cu1.5Mg) plates and and an Al5Mg wire with a diameter of 0.8 mm were selected as the base metals for the joining of aluminum/ titanium. Steel and titanium plates were cut into a size of 100×25×1 mm and aluminum plates were cut into a size of 100×25×1.5 mm, respectively. The base metals were cleaned before welding by mechanically and chemically methods. The wire nozzle was just located in front of the beam spot along the welding direction and a groove of 45° at the aluminum side was machined. The wire feeding angle was 45° and the fire feed rate was 10 mm•s-1. The experiments were carried out in a vacuum environment of 5×10-2 Pa, the beam focused on the Al side and the beam offset was 0.3 mm. The accelerating voltage was controlled in 20 kV, the beam current and welding speed were 52 mA and 2.5 mm•s-1, respectively. The process parameters of the controlled trail were the same and no groove was adapted.
The joints were incised by wire-cut electric discharge machine to prepare the metallographic and tensile specimens. Macrostructures and microstructures of the cross sections of the joints were determined by scanning electron microscopy including Energy Dispersive Spectroscopy in spot and line scanning modes. The tensile tests were carried out with the electronic universal material testing machine at the displacement velocity of 1mm/min. The fracture surfaces of the joints were observed by SEM. The X-ray diffraction (XRD) was adapted on the fractured surface to identify the interfacial IMC layer in step mode.
For the al/Fe joint, Si element existing in the filler wire can help to improve the wettability of aluminum on steel and form a continuous interfacial IMC layer of Al8Fe2Si. The interface of the joint without filler metal was characterized by a discontinuous Fe2Al5 compound layer with micro cracks in it. Meanwhile, the nano-hardness and Young’s modulus of Al8Fe2Si compound were both lower than those of Fe2Al5 compound. The tensile strength of the joint welded with filler wire was higher than that of the joint welded without filler metal. It implied that a continuous IMC layer with lower hardness is beneficial to improve the strength of the electron beam welding-brazed 304 SS to CP-Al joint.
For the Al/Ti joint, TiAl3 with lamella-shape or cellular/serration-shape formed at the interface. The interfacial morphology of EBW with filler wire was more uniform than that in controlled trail along the thickness direction. The grain boundary grooving was considered to be responsible for the forming of cellular/serration-shaped interfacial layer. The EBW process with filler wire owned uniform thermal field, which promoted the diffusion of Ti atoms at the bottom of the weld. The tensile strength can be up to 360 MPa. The reason why the joint obtained by EBW with filler wire owns higher tensile strength than that of controlled trail can be attribute to the different interfacial morphology. The crack more prone to initiated in lamella-shaped morphology reaction layer than the cellular/serration-shaped reaction layer, namely, the joints with lamella-shaped morphology reaction layer were susceptive to fracture. Thus, the joint obtained by controlled trail with the reaction layer at bottom of the joint showing lamella-shaped morphology owns lower tensile strength than that of EBW with filler wire.
As a conclusion, when a filler wire was adapted in the electron beam welding-brazing of aluminum to steel and titanium, the IMC layer was regualted into a optimized structure. Tensile strengths of joints with filler metal were improved greatly.
Key words: electron beam welding-brazing, aluminium, stainless steel, titanium, interfical layer, tensile strength